Staphylococcus aureus is one of the most commonly identified pathogens in human medicine and is a major cause of nosocomial infections and community-acquired infections. Resistance to methicillin, reported for the first time in 1961, is now widespread in hospitals all over the world. The rapid and reliable identification of methicillin-resistant Staphylococcus aureus (MRSA) has become essential for appropriate patient care, and control of strain spreading.
A specific medium is optimal when it promotes the growth of the expected species and inhibits the growth of non-expected species. For the detection or enrichment of MRSA, it is critical to get all MRSA and to inhibit all MSSA, because both micro-organisms share identical phenotypic activities. For such a detection or enrichment medium, both sensitivity and specificity must be excellent.
The standard procedure for identifying MRSA is based on cultures using selective agar media (Cherkaoui et al, 2007, J. Med. Microbiol., 56:500-503).
International patent applications WO2004/027086 and WO2004/063391 disclose chromogenic agar media containing a β-lactam antibiotic, in particular a cephalosporin. Several chromogenic media for the screening of MRSA are commercially available. Cherkaoui et al, supra, compared the performance of four of them: oxacillin-resistance screening agar base (ORSAB; Oxoid), MRSA ID (BioMérieux); Chromogen oxacillin S. aureus (Axon Lab), and MRSASelect (Bio-Rad). Stoakes et al, 2006, J. Clin. Microbiol., 44(3):637-639) compared MRSASelect to CHROMagar MRSA (Becton-Dickinson), and mannitol-salt medium supplemented with oxacillin or cefoxitin (MSA-OXA and MSA-CFOX, Oxoid). However these media are not devoid of drawbacks.
Their stability is not warranted, since cephalosporins are known to be very unstable at room temperature or at higher temperature. For a commercial standpoint, it is important that the performances of the plates or tubes do not decrease during the shipping or delivery that can take several days usually at room temperature.
Furthermore, most of these media give a result after 18-24 h of incubation. To improve the sensitivity, the user often needs to extend the incubation time (generally 48 h), which leads to a delayed diagnosis and generally to a decreased specificity, particularly when the medium is not stable enough. It is also often recommended for the user to perform confirmation tests, such antimicrobial susceptibility testing, latex agglutination or PCR, which leads to an increased cost of the analysis and, once again, a delayed diagnosis. Therefore there is a continuing need to provide a MRSA detection medium with higher performance than the current chromogenic media in terms of sensitivity and/or specificity. Preferably such a medium should allow a rapid detection (within 18-24 h) and exhibit a good stability.